1. Field of the Invention
Embodiments described herein relate generally to a fading simulator and a method of producing a fading signal.
2. Description of the Related Art
Fading occurs when a mobile communications terminal receives radio waves from a base station while moving in a propagation path in which the amplitude and phase of radio waves vary randomly. Therefore, in order to evaluate the communication performance of the mobile communications terminal, a device which simulates a radio propagation environment and is referred to as a fading simulator is used along with a pseudo base-station apparatus which simulates the base station (see JP 8-265187 A).
In recent years, mobile communications terminals have been used to more frequently download data through the Internet. Thus, as systems for increasing the amount of data to be downloaded, a multiple input multiple output (MIMO) system, a multicarrier system (for example, carrier aggregation), etc., have been proposed. Mobile communications terminals applied in the MIMO system is each provided with a multi-antenna, and receive a plurality of radio-frequency (RF) signals.
In general, in the case where an ordinary test related to a reception characteristic, etc., is conducted on a multi-antenna mobile communications terminal, using a pseudo base-station apparatus, a transmitter is prepared. The transmitter outputs as test signals, RF signals which are modulated in different basebands, and the number of which corresponds to the number of the antennas. The test signals are transmitted from the transmitter to the mobile communications terminal to conduct the test thereon.
Also, in a fading test on the multi-antenna mobile communications terminal, fading processing is performed which causes fading to occur in a baseband signal. There is a case where the baseband signal subjected to the fading processing is converted into an RF signal, and the RF signal is applied as a test signal for fading simulation. In this case, a signal generator generates a plurality of different baseband signals, not RF signals.
Incidentally, in the above ordinary test, a plurality of RF signals are output as test signals. Thus, there is a demand for conduction of a test related to fading, using the RF signals. To be more specific, it is demanded that from a transmitter which is applied to the above ordinary test to generate RF signals, the RF signals are received, and fading simulation is performed by a fading simulator which gives fading to baseband signals obtained from the received RF signals.
In this case, the above transmitter produces a digital baseband signal, converts it into an analog baseband signal, also converts the analog baseband signal into an RF signal, and outputs the RF signal as a test signal. The fading simulator receives via an RF reception module the RF signal output from the transmitter, and converts the frequency of the received RF signal to extract a baseband signal. Furthermore, the fading simulator performs predetermined fading processing on the extracted baseband signal to produce a fading signal. Then, the fading simulator converts the frequency of the fading signal into an RF band frequency to change the fading signal back to an RF signal, and outputs the RF signal as a test signal to the mobile communications terminal.
However, in a time period in which after the digital baseband signal is produced by the transmitter, it is received by the RF reception module of the fading simulator, there is a possibility that a baseband signal in which fading is to be made to occur will be influenced by the conversion at the RF band frequency, etc., and the quality of the baseband signal will thus be changed. For example, in the case where noise enters the RF reception module of the fading simulator from the outside thereof, if the performance (for example, a noise removal performance) of the RF reception module is low, the quality of the baseband signal output from the RF reception module is lowered, as a result of which a fading signal also having a low quality is produced.
As a countermeasure against the above, for example, it is conceivable that the performance of the RF reception module is improved. However, in this countermeasure, a single RF reception module is made to have a larger structure. Also, in the case where a fading test is conducted on a multi-antenna mobile communications terminal, it is necessary to provide a plurality of RF reception modules, and a fading simulator is made to have a further larger size.